Very sensitive strain sensing devices are greatly needed for monitoring a state of a structure, such as an expressway, a building, a bridge, an airplane, a ship, or the like, or for early detecting a defect that occurs in an extreme situation such as a natural disaster such as earthquake, typhoon, or the like. Therefore, piezoelectric elements such as lead zirconate titanate (PZT), ZnO, and ZnSnO3 are being researched.
However, most of piezoelectric elements are fragile ceramic materials, and are difficult to be used as a flexible strain sensor. A special manufacturing process is mainly needed for forming a piezoelectric element on a flexible substrate.
As a stimuli-responsive device, a touch sensor array in which a PVDF copolymer (a piezoelectric material) is used as a material of a sensor and a field-effect transistor (FET) is used as a signal converter is disclosed (Bauer et al., Appl. Phys. Lett. 2006, 89, 073501). In disclosed research, the touch sensor array operates in a scheme where a gate of an FET extends to thereby be connected to a piezoelectric element that is a sensor, and thus, the FET amplifies a signal at the same time with sensing.
Another research (Someya et al., PNAS, 102, 12321, 2005) discloses an array structure into which an OTFT-based pressure sensor using a piezoresistive electrode and a separate temperature sensor are integrated, and which simultaneously senses a temperature and a pressure.
However, when using a structure in which a sensor and an FET signal converter are split from each other, there are many problems such as a density of sensor pixels being limited, a manufacturing process being complicated, etc.